The present invention is generally directed to toner resins, toners thereof, and to processes for the preparation thereof. More specifically, the present invention relates to partially crosslinked resins that can be selected for the preparation of high gloss heat fixable toners with, for example, excellent low temperature fixing characteristics and superior gloss and offset properties in a hot roll fixing system, and with excellent vinyl offset properties and wherein in embodiments the fuser roll life can be increased.
A need exists for high gloss toners which melt at lower temperatures than a number of toners now used with certain copying and printing machines. Temperatures of approximately 160.degree. to 200.degree. C. are often selected to fix a toner to a support medium, such as a sheet of paper or transparency, to create a developed image. These high temperatures may reduce or minimize the life of certain fuser rolls such as those comprised of silicone rubbers or fluoroelastomers like VITON.RTM., may limit fixing speeds, and may necessitate larger amounts of power to be consumed during operation of a copier or printer, such as a xerographic copier which employs a method of fixing such as, for example, hot roll fixing.
Toner utilized in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black or other color pigment is utilized often, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are known.
To fix the toner to a support medium, such as a sheet of paper or transparency, hot roll fixing is commonly used. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll, with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium, forming a fixed image. This fixing system is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is referred to as the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images, resulting for example in blurred images. This undesirable phenomenon is known as offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is referred to as the Fusing Latitude.
Gloss performance of toner can be characterized as a function of fusing temperature as shown in FIG. 1. The fusing temperature at which the image attains a gloss level of 50 gloss units is referred to as the Gloss 50 Temperature, T(G.sub.50); hereinafter, all gloss units refer to TAPPI T480 75.degree. specular gloss unless otherwise indicated. The difference between T(G.sub.50) and HOT is referred to as the Gloss Latitude. The maximum gloss level of the image in the temperature range between MFT and HOT is referred to as Peak Gloss.
Many prior art toner resins developed have the required melt viscosity to produce images with high gloss on plain paper, for example from about 25 to about 60 gloss units, with reference to a high gloss toner resin. Toners which generate high gloss images are often selected for process color applications and transparencies. Although these properties are desired, the fixing or fusing temperature of toners prepared from these resins are high and usually more than 160.degree. C. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Offsetting can also be a problem. Furthermore, toners containing vinyl type binder resins, such as styrene-acrylic resins, may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface. Also, a number of toner resins having lower melt temperatures have a narrow fusing latitude and have poor mechanical properties, creating too many fines during jetting which have to be removed by classification and reused. This results in increased cost of the toner.
There is a need for a high gloss toner resin and toner thereof, which has a fix temperature below 200.degree. C., preferably below 160.degree. C. (referred to as low fix temperature toner resin or low melt toner resin), excellent offset performance, wide gloss latitude, and superior vinyl offset properties, and processes for the preparation of such a resin. Toners which operate at lower temperatures would reduce the power needed for operation and increase the life of the fuser roll and the high temperature fuser roll bearings. Additionally, such low melt toner resins would reduce the volatilization of release oil, such as silicon oil, which may occur during high temperature operation and which can cause problems when the volatilized oil condenses in other areas of the machine. In particular, high gloss toners with a wide fusing and excellent gloss latitude and with good toner particle elasticity are needed. Further, toners with wide fusing and excellent gloss latitude can provide flexibility in the amount of oil needed as release agent, can minimize copy quality deterioration related to the toner offsetting to the fuser roll and can extend fuser roll life. These and other advantages are achievable with the toners and processes of the present invention.
To lower the minimum fix temperature of the binder resin, in some instances the molecular weight of the resin may be lowered. Low molecular weight amorphous polyester resins and epoxy resins have been used for low temperature fixing toners. For example, attempts to use polyester resins as a binder for toner are disclosed in U.S. Pat. Nos. 3,590,000 and 3,681,106. The minimum fixing temperature of polyester binder resins can be lower than that of other materials, such as styrene-acrylic and styrene-methacrylic resins. However, this may lead to a lowering of the hot offset temperature, and as a result, decreased offset resistance and shortened fuser roll life. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage. Furthermore, toner prepared from such a resin will usually generate images with undesirable crease performance and narrow fusing latitude.
U.S. Pat. No. 5,057,392 discloses a low fusing temperature toner powder which employs a polyblend of a crystalline polyester and an amorphous polyester that has been crosslinked with an epoxy novolac resin in the presence of a crosslinking catalyst. The disclosed polyblend contains a mechanical mixture of the crystalline and amorphous polyester melt blended together. The crystalline polyester is required to maintain a desired low melt temperature and the amorphous polyester is required to maintain a desired high offset temperature. In the polyblend, the amorphous polyester is partially crosslinked with the epoxy novolac resin. The disclosed toner powder cannot be achieved in the absence of crystalline and amorphous polyesters, and upon completion of crosslinking, the crystalline polyester recrystallizes as dispersed small particles within a matrix phase of the crosslinked amorphous polyester and epoxy resin. In one disclosed process for preparing the toner particles, the crystalline polyester, amorphous polyester resin, epoxy novolac resin, crosslinking catalyst, colorant, crystallization promoter and optional charge control agent are melt blended, preferably by an extrusion process. During melt blending, the amorphous polyester is crosslinked with the epoxy novolac resin. After melt blending, the mixture is annealed to recrystallize the crystalline polyester. The disclosed melt blended mixture is not useful as a toner particle requiring a low melt temperature until it is annealed. In addition, the glossy image generated on paper with toner prepared from such a mixture does not have a wide fusing latitude.
To prevent fuser roll offsetting and to increase fuser latitude of toners, various modifications have been made to toner compositions. For example, waxes, such as low molecular weight polyethylene, polypropylene, and the like, have been added to toners to increase the release properties as disclosed in U.S. Pat. No. 4,513,074, the disclosure of which is totally incorporated herein by reference. However, to prevent offset sufficiently, considerable amounts of such materials may be required in some instances, resulting in detrimental effects such as the tendency to toner agglomeration, worsening of free flow properties and destabilization of charging properties. Also, waxes tend to degrade projection efficiency of glossy color transparencies.
Modification of binder resin structure, for example, by branching and crosslinking when using conventional polymerization reactions, may also improve offset resistance. In U.S. Pat. No. 3,681,106, for example, a polyester resin was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
U.S. Pat. No. 4,797,339 discloses a modified toner resin containing a particle-to-particle ionically crosslinked resin complex. The disclosed crosslinked resin complex is obtained by reacting a cationic resin emulsion and an anionic resin emulsion. The resulting resin ion complex has a glass transition temperature of -90.degree. C. to 100.degree. C. and a degree of gellation of from 0.5 to 50 percent by weight, and preferably 10 to 30 percent by weight. It is indicated in this patent that if the degree of gellation is too high beyond 50 percent by weight, the fixability of the toner at low temperatures tends to be reduced undesirably. If it is too low below 0.5 percent by weight, scattering of the toner tends to increase undesirably. The emulsion polymerization process disclosed results in production of a sol component in the polymer, that is crosslinked portions which are not densely crosslinked.
A method of improving offset resistance of high gloss resin is to utilize crosslinked resin in the binder resin. For example, U.S. Pat. No. 3,681,106 discloses a toner in which a crosslinked polyester, prepared using conventional crosslinking methods, is used as the binder resin. Similar disclosures for polyester resins are made in U.S. Pat. Nos. 4,933,252 and 4,804,622.
While significant improvements can be obtained in offset resistance and entanglement resistance in glossy toner resins, a major drawback may ensue in that with crosslinked resins prepared by conventional polymerization, that is crosslinking during polymerization using monomer and a crosslinking agent, there exist three types of polymer configurations: a linear and soluble portion referred to as the linear portion, a portion comprising highly crosslinked gel particles which is not soluble in substantially any solvent, e.g., tetrahydrofuran, toluene and the like, and is the gel, and a crosslinked portion, which is low in crosslinking density and, therefore, is soluble in some solvents, e.g., tetrahydrofuran, toluene and the like, and is the sol. Also, there are monomeric units between the crosslinked polymer chains. The presence of highly crosslinked gel in the binder resin increases the hot offset temperature, but at the same time the low crosslink density portion or sol increases the minimum fix temperature. An increase in the amount of crosslinking in these types of resins results in an increase not only of the gel content, but also of the amount of sol or soluble crosslinked polymer with low degree of crosslinking in the mixture. This results in an elevation of the minimum fix temperature, and as a consequence, in a reduction or reduced increase of the fusing latitude. In addition, a drawback of embodiments of crosslinked polymers prepared by conventional polycondensation in a reactor with low shear mixing, for example less than 0.1 kW-hr/kg, is that as the degree of crosslinking increases, the gel particles or very highly crosslinked insoluble polymer with high molecular weight grow larger. The large gel particles can be more difficult to disperse pigment in, causing the formation of unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, compatibility with other binder resins may be relatively poor and toners containing vinyl polymers often show vinyl offset.
U.S. Pat. No. 4,533,614 discloses a loosened crosslinked polyester binder resin which shows low temperature fix and good offset resistance. Metal compounds were used as crosslinking agents. Similar disclosures are presented in U.S. Pat. No. 3,681,106 and Japanese Laid-Open Patent Applications 94362/1981, 116041/1981 and 166651/1980. As discussed in the '614 patent, incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner. Also, in the case of color toners other than black (e.g., cyan), metal complexes can adversely affect the color of pigments. It is also known that metal containing toner can have disposal problems in some geographical areas, such as for example in the State of California, U.S.A. Metal complexes are often also expensive materials.
Many processes are known for effecting polymerization reactions, including reactive extrusion processes, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modification reactions, such as graft, coupling, crosslinking and degradation reactions.
U.S. Pat. Nos. 4,894,308 and 4,973,439, for example, disclose extrusion processes for preparing electrophotographic toner compositions in which pigment and charge control additive were dispersed into the binder resin in the extruder. However, in each of these patents, there is no suggestion of a chemical reaction occurring during extrusion.
An injection molding process for producing crosslinked synthetic resin molded articles is disclosed in U.S. Pat. No. 3,876,736 in which polyolefin or polyvinyl chloride resin and crosslinking agent were mixed in an extruder, and then introduced into an externally heated reaction chamber outside the extruder wherein the crosslinking reaction occurred at increased temperature and pressure, and at low or zero shear.
In U.S. Pat. No. 4,089,917, an injection molding and crosslinking process is disclosed in which polyethylene resin and crosslinking agent were mixed in an extruder and reacted in reaction chambers at elevated temperature and pressure. Heating of the resin mixture occurred partially by high shear in inlet flow orifices. However, the crosslinking reaction in this process still took place in the reaction chambers at low or zero shear, and the final product is a thermoset molded part, and thus is not useful for toner resins.
A process for dispensing premixed reactive precursor polymer mixtures through a die for the purposes of reaction injection molding or coating is described in U.S. Pat. No. 4,990,293 in which polyurethane precursor systems were crosslinked in the die and not in the extruder. The dimensions of the die channel were determined such that the value of the wall shear stress was greater than a critical value in order to prevent gel buildup and consequent plugging of the die. The final product is a thermoset molded part, and thus is not useful for toner resins.
The processes disclosed in U.S. Pat. Nos. 3,876,736; 4,089,917 and 4,990,293 are not considered reactive extrusion processes because the crosslinking in each case occurs in a die or a mold, and not in an extruder, and the crosslinking takes place at low or zero shear. These processes are for producing engineering plastics such as thermoset materials which cannot be remelted once molded, and thus are not useful in toner applications.
In U.S. Pat. No. 5,395,723, a polyester toner resin is described, which is prepared by reactive extrusion, and which is suitable for low gloss matte application, such as for example matte black and highlight color application, and which has low fix temperature, excellent offset resistance, wide fusing latitude and shows minimized or substantially no vinyl offset. Also, in U.S. Pat. No. 5,227,460 there are disclosed low melt toners with reactive extruded resins and wherein the microgel particles can be present in an amount of from about 0.001 to about 50 percent, and more specifically, 0.7 weight percent, reference Example I, column 16, and column 7, lines 10 to 15, wherein gel content is recited as about 0.001 to about 50, 0.1 to 40, or 10 to 19. The disclosures of each of the aforementioned documents are totally incorporated herein by reference.
There is a need for high gloss, low melt toner resins and toners thereof with excellent offset resistance, wide fusing and broad gloss latitude, and which resin and toner thereof possess minimized or substantially no vinyl offset, and which toner can be used for process color applications and transparencies. The toner resins involved possess different rheological properties than a resin selected for low gloss applications.